Acoustic waves, i.e., compression and expansion waves, travelling through solid and liquid materials are known to cause deformations in the materials at the molecular level. These deformations may affect other properties of the materials. For instance, materials which are transparent at certain frequencies of electromagnetic radiation, such as visible light, have an index of refraction. The indices of refraction of two different materials which abut each other may be used to calculate an angle of refraction of an incident beam of light or other radiation as it passes between the materials. The index of refraction of a material is one property of the material which may be affected by acoustic waves passing through the materials.
Acoustic waves may be generated in a piece of material, such as a crystal, electronically. A thin-film plate or piezoelectric transducer fixed to a side of the crystal converts an electrical signal into a mechanical vibration. This vibration propagates from the side through the crystal as an acoustic wave.
A beam of electromagnetic radiation, such as a laser beam, passing through the crystal is diffracted or refracted as though the crystal propagating acoustic waves were a diffraction grating or a prism. The degree of diffraction or refraction may be related to factors such as the magnitude, wavelength, etc., of the acoustic waves. Thus, the laser beam may be modulated as a function of the signal.
A very high frequency signal may be provided to the transducer. That signal itself may be amplitude- or frequency-modulated with a lower frequency signal. Thus, the laser output may be modulated with information over a wide range of frequencies such as from a few hertz to up to 100 MHz.
Devices including a block of transparent material having transducers mounted on their sides have been made for operation as described above. These conventional acousto-optical devices have been implemented using fused quartz or fused silica blocks. While these devices have been used in many applications, various properties inherent in the fused silica and fused quartz have limited their usefulness. These properties include the following:
1. Acoustic attenuation (should be as low as possible) PA0 2. Thermal conductivity (should be as high as possible) PA0 3. Q (should be as low as possible) PA0 4. Loss modulation efficiency (should be as high as possible) PA0 5. Drive power efficiency (should be as high as possible) PA0 6. Angular sensitivity (should be as low as possible)
Limitations as to these properties have limited the usefulness and applications of acousto-optical devices.